The present invention relates to a spring steel having a good sag-resistance.
There has been an increasing demand for light weight suspension springs reflecting a trend for light weight automobiles, in recent years. As an attempt to meet such a demand, it is said to be an effective approach to the reduction of weight to design the springs to have an increased stress and to use them under a high stress state.
However, if presently available spring steels are used under a high stress condition, there will arise problems such as deterioration of their durability and increase of sagging, and the increased sagging will result in decreased height of the springs and hence decreased height of the vehicle, with the consequent decreased height of the bumper causing a serious problem from the standpoint of safety.
Under the circumstances, there has recently been a demand for a spring steel having a high sag-resistance which makes high stress designing possible.
Heretofore, as a spring steel superior in sag-resistance, the steel corresponding to SAE 9260 (Japan Industrial Standard SUP 7) has become more popular along with the finding that silicon contained in spring steels is effective in improving sag-resistance. However, there were severe requirements for light weight suspension springs. Accordingly, it has been strongly desired to develop a spring steel having a sag-resistance superior to that of SAE 9260.
With these circumstances as background the inventors of the present invention have previously developed a spring steel superior in the sag-resistance to the steel of SAE 9260 and equivalent to the steel of SAE 9260 in the fatigue resistance and toughness required of spring steels, by adding one or more of vanadium, niobium and molybdenum in an appropriate amount to a spring steel of high silicon content, and filed an application thereon ( U.S. patent application Ser. No. 06/289,852).
In manufacturing such a high silicon content spring steel, it is sometimes required to perform the so-called reladling operation involving pouring molten steel into a ladle and then transferring it into another ladle, which operation results in increased cost. Moreover, it is known that an increased silicon content promotes decarburization of the steel surface, and particularly in the case of using the steel as rolled, it is necessary to exercise an ample care in its manufacture.